Portfolio rebalancing is the process of bringing a portfolio back toward its intended asset allocation after market movements cause the weights to drift. The purpose is risk control, not market prediction.
There is no universal rule that every investor should rebalance on the same date or at the same percentage of drift. A useful policy defines in advance when the portfolio will be reviewed, what amount of deviation matters and how transaction costs or taxes will be considered.
Why portfolios drift
Suppose a portfolio starts with different target weights for equities, bonds and cash. If equities rise faster than the other assets, the equity share can become larger than intended. The portfolio may then carry more equity risk than the original plan allowed.
The opposite can also happen after a large decline. Rebalancing is the decision to restore the intended structure rather than allowing recent performance to rewrite the portfolio automatically.
How often should a portfolio be rebalanced?
There is no single correct rebalancing frequency. Investor.gov describes two common approaches: reviewing the portfolio on a calendar schedule, with six- or twelve-month intervals given as examples, or rebalancing when an asset class moves beyond a predefined threshold.
Those examples are not universal mandates. A policy should fit the portfolio's size, tax environment, transaction costs, liquidity and the consequences of allowing the allocation to drift.
Calendar and threshold policies solve different problems
A calendar policy creates a regular decision point. The investor reviews the portfolio at a predetermined interval and decides whether the current weights require action.
A threshold policy lets the amount of drift trigger the review. This can respond sooner after a large market move while avoiding trades when the allocation remains close to target. The threshold itself should be chosen before the market move rather than invented after seeing which asset performed best.
New contributions can rebalance without selling
An investor who is adding money regularly can direct more of the new capital toward underweight assets. This can reduce drift without selling an overweight holding immediately.
That method can be useful where selling would create transaction costs or tax consequences, although the actual tax treatment depends on the investor's jurisdiction and account type.
Rebalancing is not the same as changing the strategy
If the investor's goal, time horizon, financial situation or risk capacity changes, the target asset allocation itself may need to be reconsidered. That is a strategic change.
Rebalancing assumes that the target policy is still appropriate and corrects the portfolio back toward it. It should not be used as a mechanical reason to preserve an allocation whose underlying objective has changed.
Costs and taxes belong inside the rule
A rebalancing trade can create spreads, commissions, market impact or taxable gains. Ignoring those frictions can turn a small theoretical improvement in allocation into unnecessary activity.
This is why rebalancing is usually a policy problem rather than a frequency contest. More frequent intervention is not automatically better.
Rebalancing keeps risk ownership explicit
Capital allocation defines what the money is for. Asset allocation defines the intended portfolio mix. Diversification reduces avoidable concentration inside that structure. Rebalancing is the maintenance process that prevents market drift from silently changing it.
A sound rebalancing policy tells you when to review, what deviation matters and how to restore the portfolio without pretending that the trigger predicts the next market move.